Measuring the spin magnetic response is an important characterization tool of superconductors, both conventional and unconventional.[1]}, [2]}
The usual assumption is that the Zeeman interaction of the conduction electrons with an external magnetic field has essentially the same form as for bare electrons, i.e., is independent of the wave vector.
Then, the temperature behaviour of the spin susceptibility depends only on the superconducting gap structure, but not on the character of the electron Bloch bands.
If the superconducting pairing occurs in the spin-singlet channel, then the susceptibility tensor
\(\chi _{ij}(T)=\chi (T)\delta _{ij}\)  is entirely determined by the thermally excited quasiparticles and
in a fully gapped superconducting state one would see an exponentially decreasing \(\chi (T)\)  at low temperatures. In contrast, if the gap has zeros on the Fermi surface, then \(\chi (T)\propto T^2\)  for isolated first-order point
nodes, or \(\chi (T)\propto T\)  for line nodes or second-order point nodes. In the triplet case, the susceptibility acquires a nontrivial tensor structure depending on the mutual orientation of the order parameter \({d}\) 
and the external magnetic field \( {H} \) . For \({d}\parallel  {H} \)  the spin response is determined only by the excitations and the susceptibility has the same temperature dependence as in the singlet case,
whereas for \({d}\perp  {H} \)  both the Cooper pairs and the excitations contribute to the spin response and the susceptibility is unchanged compared to the normal state.
